Merge branch 'hotfix/3679' into develop

Forward port zendframework/zendframework#3679

src/Headers.php

@@ -126,7 +126,7 @@ public function getPluginClassLoader()
      */
     public function addHeaders($headers)
     {
-        if (!is_array($headers) && !$headers instanceof \Traversable) {
+        if (!is_array($headers) && !$headers instanceof Traversable) {
             throw new Exception\InvalidArgumentException(sprintf(
                 'Expected array or Traversable; received "%s"',
                 (is_object($headers) ? get_class($headers) : gettype($headers))